It's mainly due to squad depth, specifically a lack of a decent second striker.
Defensively we are better then people's perceptions of us. It's because we have had a few high profile defensive debacles, but the statistics don't point to the defense being a major problem. Added to that people will always compare the current defense to the Graham era defense that was notoriously good. Also Vermaelen by far our best defender has been out the whole season and I think most teams would drop points without their star defender. Look at Chelsea when Terry was injured and they have a great defense.
There is also an element of psychology in the team that means we don't speculate enough while attacking. We score far to few goals from long and medium range because there is a reluctance to pull the trigger if there is a forward pass available. This is tied up with Arsenals style of football, but it also means players tend not to take responsibility for missing the opportunity to shoot. Players have said in the passed that sometimes the team lets opportunities pass them by because they won't take enough personal responsibility for finishing. We do tend to look to Fabregas and Van Persie to finish opportunities or play a final ball.
I think it's not really Wengers priority to win trophies no matter what he says. I think his priority is developing the facilities, finances, scouting, stadium and youth development. Trophies and commercial development are secondary. Arsenal are notoriously under commercial when compared to the other big 4 Premiership team. I saw Wenger in an interview once quite candidly laying out the relative returns for being successful in each area of a football club. He made it clear that he wouldn't over invest in the top wages and fees to bring trophy winning players to guarantee victory. He said if he can be competitive without adding debt to the club he would continue on that path.
There are also the small dramas of the club. Things like Fabregas/Barca saga that has reduced the effectiveness of our more talented players. The long world cup campaigns of both Spain and the Netherlands added to Fabregas and Van Persie's mixed season. The injuries of Van Persie, Walcott and Vermaelen didn't help, no team this year because the league is so competitive has done well with key injuries. Then you have weird contract stuff with senior players like Clichy and Nasri. I think next season there will be more stability. hopefully Fabregas will leave and Vermaelen will be fit enough to become captain. We should also sell some 21+ year old squad players and use the money to buy players in more key to winning a title. Denilson, Bendter, Almunia, Rosicky, etc should all go this season. Maybe even a few others might want to go Eboue, Clichy and Arshavin being the most likely.